Among ceramic composites comprising metal carbides, carbon fiber reinforced SiC ceramics, which include the C—C/SiC (composites reinforced with carbon fibers and comprising silicon carbide and carbon in the matrix) and C/SiC material systems, have in recent times achieved particular industrial importance.
As preferred process for producing fiber-reinforced SiC, C—C/SiC or C/SiC composites, liquid silicon infiltration (LSI) has been found to be particularly advantageous for numerous applications such as brake and clutch discs, satellite mirrors and combustion chamber linings.
The LSI process comprises the following significant steps:                a) production of a CFRP (=carbon fiber reinforced polymer) intermediate body,        b) carbonization of the intermediate body to give a porous carbon-containing green body, typically a C/C body (carbon fiber reinforced carbon),        c) melt infiltration of the green body with a silicon melt,        d) reaction of at least part of the carbon of the green body with the silicon to form SiC, giving a carbon fiber reinforced composite ceramic having a matrix comprising SiC, Si and C.        
Such a process is known, for example, from DE-A 197 10 105.
Composites having matrices comprising other metal carbides can be produced in an analogous manner by melt infiltration with the appropriate metal carbide-forming metals. Both the semimetals silicon and boron and the metals iron, nickel, titanium, zirconium, vanadium, chromium, molybdenum and tungsten and any alloys of these metals and semimetals are of importance here. In the interests of simplicity, the abovementioned semimetals will hereinafter be referred to as metals, too.
The technically simplest method of supplying the metal melt is to cover the porous green body with a bed of metal powder, in particular silicon powder, which becomes liquid on heating above its melting point and is taken up into the pores of the green body by capillary action. However, this simple process has the serious disadvantage that the sometimes very fluid melt can flow unhindered down from the green body and away without infiltration taking place, i.e. without penetration of the melt into the hollow spaces in the interior of the green body. The uptake of metal can therefore be controlled only with difficulty. In particular, special measures have to be taken to prevent the body which is coated and/or impregnated with metal from becoming firmly joined (soldered) to the crucible or the substrate by means of a connecting metal layer.
These problems can be avoided when the metal, in particular silicon, is present in the form of a shaped body (known as silicon donor shaped body) or when significant proportions of the metal melt are supplied via porous substrates or wick materials.
For the present purposes, “wicks” are porous bodies which draw in liquids by capillary action and can release them at another point. Taking off the liquid at this other point enables transport of the liquid through the wick to take place.
Thus, for example, in EP-A 0 995 730, a process is proposed for producing silicized shaped bodies in which silicon powder or silicon granules are combined with a binder and shaped to give a shaped body and the green body is infiltrated with the silicon which, at an appropriate temperature, flows downward from the silicon donor shaped body onto the green body. However, this process entails the risk that the residues of the silicon donor shaped body sinter onto or conglutinate with the silicized composite and can be removed completely only by means of costly after-treatment of the surface.
One way of supplying liquid silicon via porous substrates and wicks is described in DE-A 197 11 831. Instead of placing the shaped body directly in a graphite crucible coated with boron nitride, it is also possible to use, as an alternative, a porous SiC charging plate which stands on feet in the silicon melt or is connected thereto via porous wicks. A relatively large amount of granulated silicon can be introduced into the crucible to produce the melt, since the melt rises from below via the porous feet and the porous charging plate into the shaped body. Although the silicon can be supplied efficiently in this way, in particular by use of porous wicks, the problem of conglutination of the composite with the SiC charging plate is not solved satisfactorily. In addition, the overall construction comprising wick and plate uses a lot of material and is costly, and is therefore unacceptable for mass production.
Furthermore, the use of carbon felts as wick material and placing the green body directly on the wicks dipping into a silicon melt is also known. The carbon felts have to have sufficient strength combined with a very high porosity. Such felts are disproportionately expensive, particularly for mass production.